1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube of a multineck structure.
2. Description of the Related Art
Color cathode ray tubes in which a fluorescent screen in a vacuum envelope is scanned by electron beams and an image is displayed on the screen, are conventionally used as high definition broadcasting devices or computer terminal high resolution graphic display devices. For these applications, increased resolution is desirable. High resolution in a color cathode ray tube can be achieved by minimizing an electron beam spot on its screen. Conventional tubes have been improved by elongating the electron gun assembly and enlarging its diameter. These improvements have not been completely satisfactory because as the diameter of the tube is increased, the distance between the electron gun assembly and the screen also increases, resulting in undesirably large magnification of the electron lens. In other words, in order to achieve high resolution, it is important to shorten the distance between the electron gun assembly and the screen. For this purpose, the tube may be designed as a wide angle deflection tube. However, in such a tube, the magnification at the center region of the screen differs from that at the peripheral region thereof.
In order to solve the above problem, Japanese Patent Disclosure (Kokai) No. 48-90428 describes a multitube structure display device having a plurality of small or medium cathode ray tubes arranged in the horizontal or vertical direction to display an image on a large screen with high resolution.
A conventional display device of the multi-tube structure can be effectively used outdoors to display an image on a very large screen divided into blocks.
However, such a display device is not suitable for a medium-size screen, i.e., about 40 inches, since the joints of the divided blocks of the screen stand out and result in a poor image. In particular, when this display device is used as a computer-aided design graphic terminal, the presence of joints is a significant shortcoming.
In addition, U.S. Pat. No. 3,071,706, Japanese Utility Model Publication (Kokoku) No. 39-25641, and Japanese Patent Publication (Kokoku) No. 42-4928 and No. 50-17167 describe a multi-tube structure in which a plurality of independent tubes are integrated into a screen. However, in such an arrangement, when a screen is divided into a plurality of separately scanned segment regions, rasters in the adjacent segment regions overlap each other at their boundaries or have a blank therebetween and result in a poor image.
In order to solve this problem, Japanese Patent Disclosure (Koki) No. 61-256551 (U.S. Pat. No. 4,714,856) describes a color cathode ray tube having a multineck structure. As shown in FIG. 1, the color cathode ray tube comprises a vacuum envelope having a single faceplate 1, on which a phosphor screen 2 is formed, and a plurality of necks 3a, . . . 3d. A plurality of electron gun assemblies 4a, . . . 4d are respectively received in necks 3a, . . . 3d. Phosphor screen 2 is defined by a plurality of continuous segment regions each of which is scanned with electron beams 5R, 5G, and 5B from a corresponding electron gun assembly. A shadow mask 6 is received in the panel and faces the screen 2. The shadow mask 6 has a plurality of effective row and column regions 7 corresponding to the segment regions and a non-effective region 8 for surrounding and partitioning the respective segment regions. A plurality of deflection units 9a, . . . 9d, generating deflection magnetic fields, are respectively mounted in the vicinity of the electron gun assemblies 4a, . . . 4b to deflect the electron beams 5R, 5G, and 5B.
Generally, in a color cathode ray tube with a shadow mask, only 30% or less of the electron beams originally emitted by the electron guns pass through the apertures formed in the shadow mask. The remaining 70% or more of the beams impinge on the shadow mask, which may become heated. As a result, the shadow mask thermally expands and deforms. When it deforms, landing error occurs and color purity is degraded. In conventional color cathode ray tubes with one phosphor screen and one electron gun assembly, deformation of the shadow mask is symmetric about the center of the phosphor screen corresponding to the center of the shadow mask. In order to correct landing error, a means for adjusting the distance between mask and screen in response to the temperature of the shadow mask and the mask frame has been used.
In a color cathode ray tube having a multineck structure, as mentioned above, deformation is symmetric about the center of the shadow mask, similar to conventional tubes. As a result, the direction of deformation of each effective region relative to the corresponding segment region is different in accordance with its position in the shadow mask. Therefore, compensation of the landing error as used in a conventional tube is not effective in this type of tube.